1. Field of the Invention
The present invention relates to a method of controlling a communication network such as an ATM (Asynchronous Transfer Mode) network which incorporates a number of VP (virtual path) exchange nodes and VC (virtual channel) exchange nodes.
2. Description of the Background Art
In recent years, progress has been made for developing an ATM network as a network architecture for realizing a broad band company integrating network and a high speed and broad band data communication in a form of B-ISDN (broadband-integrated service digital network) system. In the ATM network, data are transmitted in units of cells, and each cell has a header containing a VCI (virtual channel identifier) for identifying a connection between end-users and a VPI (virtual path identifier) for identifying a logical communication path, such that the desired transfer of the cell is achieved at a number of exchange nodes according to these VCI and VPI.
There are two types of exchange nodes in the ATM network, including a VC exchange node which carries out the exchange operation for each VC according to the VCI, and a VP exchange node which carries out the exchange operation for each VP according to the VPI. The VP is set up between the VC exchange nodes, and provides a direct logical path between the VC exchange nodes through one or more VP exchange nodes. The VP exchange node carries out the exchange operation in units of VP by using the VPI only regardless of the VCI, while the VC exchange nodes carries out the exchange operation in units of VC by using both the VCI and the VPI. The set up control for VC including operations such as an admission control operation and a routing table renewal operation is carried out only at the VC exchange nodes.
A conventional ATM network using both of the VC exchange nodes and the VP exchange nodes has a schematic configuration shown in FIG. 1, where the network includes: a VC exchange node 101 for receiving the VCs from a user terminal 100 at which the admission of the VCs is determined and the admitted VCs are multiplexed and outputted to VPs; a VP exchange node 111 for receiving the VPs from the VC exchange node 101 and multiplexing and outputting them to a transmission path 121; a VP exchange node 112 for receiving the multiplexed VPs from the transmission path 121 and switching them into a transmission path 122 as well as other transmission paths (not shown); a VP exchange node 113 for receiving the multiplexed VPs from the transmission path 122 and separating and outputting them; and a VC exchange node 102 at which the VCs are separated from the VPs received from the VP exchange node 113 and transmitted to another user terminal (not shown).
As shown in FIG. 1, the VC exchange node 101 has: an input port 131 at which the cell flow for each VC arriving From the user terminal 100 is monitored; an ATM switch 132 For performing a switching operation on the VCs received by the input port 131; an output port 133 for performing the priority control at its buffer (when the priority control is necessary), multiplexing the VCs outputted by the ATM switch 132 into VPs and outputting them to a transmission path: and a bandwidth managing unit 134 for performing the VC admission control in response to the VC set up requests from the user terminal 100 in which the number of VCs admitted at the input port 131 and the allocation of the VCs to be outputted at the output port 133 are controlled in order to secure the required quality of service. Also, separate controlling in units of VP is provided at each of the VP exchange nodes 111,112, and 118.
Now, in the ATM network, it is preferable to achieve a highest transmission bandwidth utilization efficiency within a limit of maintaining a prescribed cell transfer quality. To this end, it is necessary for the network to carry out an appropriate bandwidth allocation control by constantly comprehending the cell transfer qualities in the network which depend on a loading state of the network at each moment.
In the VP based ATM network, two levels of such a bandwidth allocation control will be necessary. Namely, a control of the allocation of the VC to the VP, and a control of the allocation of the VP to the physical transmission path. The former is a control performed for each call (each VC) for which only the VC exchange nodes will be relevant. The latter is a control performed on a basis of a relatively long term traffic demand, in which the VP capacity and the route can be changed according to the loading state of the network.
In order to carry out these bandwidth allocation controls appropriately, it is necessary to know what kind of influence each level of bandwidth allocation has on the cell transfer quality in the network, where the following facts should be accounted for.
In carrying out the allocation of the VC to the VP, it becomes necessary to estimate the cell transfer quality resulting from the multiplexing of the VC into the VP according to the prescribed bandwidth (cell flow) specified to each VP and the bandwidth (cell flow) specified for each VC. However, the VP will be further multiplexed into the transmission path, so that in the actual cell transfer phase the cell transfer quality will be affected by the cell flows of the other VPs which are multiplexed together.
On the other hand, the bandwidth (cell flow) of each VP is controlled to be within the prescribed bandwidth by the control of the allocation of the VC to the VP. However, in the actual cell transfer phase, the cell flows of various VCs belonging to various VPs transmitted from a number of input transmission paths will be switched to the output transmission path of a target VP, such that there is a possibility for violating the prescribed bandwidth specified to the target VP depending on the arrival timing of the cell flows arriving at the target VP from the different input transmission paths.
For example, as shown in FIG. 2, when the prescribed minimum cell interval in the VP is 10 cell time, the cells which are evenly distributed within the 100 cell time such as those of the VP1 obtained from the VC1 to VC10 (prescribed minimum cell interval for each of these VCs is set to be 100 cell time) cause no problem, but the cells which are unevenly distributed within the 100 cell time such as those of the VP10 obtained from the VC91 to VC100 (prescribed minimum cell interval for each of these VCs is set to be 100 cell time) will cause a problem as they are actually beyond the given capacity of the VP, and this will eventually affect the cell transfer qualities of the other VPs as they are multiplexed together on the transmission path.
In addition, in a case that the allocation of the VC to the VP is carried out by allowing the bandwidth of the VP to exceed the prescribed bandwidth temporarily on a basis of the statistical multiplexing effect, there is a possibility for violating the prescribed bandwidth specified to the target VP regardless of the arrival timing of the cell flows arriving at the target VP from the different input transmission paths.
Now, in a case that the actual cell flow of a certain VP is violating the bandwidth prescribed to this VP, such a VP can statistically influence the other VPs which shares the same transmission path with that VP beyond the limit of the influence estimated at a time each of these other VPs is allocated to that transmission path, such that there is a danger of causing the lower cell transfer qualities for the other VPs than expected.
In order to avoid such a problematic situation, it is necessary to regularly monitor the cell flow at each VP and carry out a transfer control capable of maintaining the cell flow of each VP within the prescribed bandwidth. Moreover, it is necessary to establish the method of specifying the cell transfer quality at the VC exchange node and the VP exchange node as well as the method of estimating the end-to-end cell transfer quality for VCs in such a transfer control.
However, in a conventional ATM network, a need for such a transfer control has been neglected and therefore such a method of specifying the cell transfer quality at the VC exchange node and the VP exchange node and a method of estimating the end-to-end cell transfer quality for the VCs in such a transfer control have not been established.